1. Field of the Invention:
The present invention relates to the ammoxidation of saturated hydrocarbons, and, more especially, to the ammoxidation/conversion of alkanes into a mixture containing .alpha.,.beta.-unsaturated nitriles.
2. Description of the Prior Art:
The ammoxidation of olefins and, in particular, of propylene, is well known to this art. However, although the saturated hydrocarbons, which are more widely available, are the more desirable starting materials from an economic standpoint, it is also well known to this art that they do not display comparable reactivity in this type of reaction to form, especially, .alpha.,.beta.-unsaturated nitriles.
One of the difficulties encountered in the ammoxidation of saturated hydrocarbons resides is the requirement for catalysts capable of dehydrogenating the saturated hydrocarbon under conditions which minimize or eliminate the combustion of the ammonia and/or that of the hydrocarbon, while at the same time ensuring a reasonable selectivity either for the .alpha.,.beta.-unsaturated nitrile (target compound), for example for acrylonitrile starting from propane, or for added value compounds (above-mentioned nitrile and olefin), for example, for acrylonitrile and propylene starting from propane.
U.S. Pat. No. 3,365,482 describes the ammoxidation, especially of isobutane into methacrylonitrile, on a molybdenum-based catalyst deposited onto eta-alumina, doped with antimony, at 508.degree. C., starting from a gaseous mixture containing isobutane, air, ammonia and steam (1.0/4.5/1.0/12.5); the selectivity for methacrylonitrile attains a value of 49% for a degree of conversion of the isobutane of 22%.
When the starting material is a gaseous mixture of propane/air/ammonia/steam (1.0/4.7/0.67/12.8), using the same catalyst and at 550.degree. C., the selectivity for acrylonitrile decreases to 15% for a degree of conversion of the propane of 29%.
In Chemistry Letters, pp. 2173-2176 (1989), the ammoxidation of propane in the vapor phase is described, in the presence of multicomponent metal oxides containing molybdenum and bismuth and having a structure of the type of that of scheelite. It appears that, despite the relatively moderate temperatures used, the proportion of combustion products (CO, CO.sub.2) is very high in all instances (at least 15%) and that certain catalytic compositions tested exhibit very little activity with respect to the desired reaction, while being used under conditions which are in the explosive region or very near the explosive region.
It is immediately apparent that the coproduction of large amounts of CO and CO.sub.2 is undesirable on an industrial scale.
In addition, the use of reaction mixtures which are in the explosive region, compositionally, is even less desirable on an industrial scale, since the process is used in a stationary bed.
U.S. Pat. No. 5,008,427 describes a process for the ammoxidation of propane or isobutane into an .alpha.,.beta.-unsaturated mononitrile, by reaction in the vapor phase with oxygen and ammonia, in the presence of a catalyst comprising vanadium, antimony, oxygen and titanium and/or tin and/or iron and/or chromium and/or gallium and, optionally, one or more other elements selected from among 23 very diverse metals. Other than the composition of the catalyst, the principal characteristics of such process are the calcination temperature of the catalyst, which must be at least 780.degree. C., and the alkane/ammonia (from 2.5 to 16) and alkane/oxygen (from 1 to 10) molar ratios.
This process would thus permit obtaining good productivity in respect of unsaturated nitriles employing a catalyst which does not agglomerate.
However, this '427 patent, which is thus essentially oriented in the direction of particular operating conditions such as calcination temperature or reactant ratios, does not describe selected compositions containing an active phase which permits attaining a good selectivity for the unsaturated nitriles.